diff --git a/scripts/mbedtls_dev/bignum_common.py b/scripts/mbedtls_dev/bignum_common.py
index 0339b1a..dd4fc36 100644
--- a/scripts/mbedtls_dev/bignum_common.py
+++ b/scripts/mbedtls_dev/bignum_common.py
@@ -39,6 +39,11 @@
         return b
     raise ValueError("Not invertible")
 
+def invmod_positive(a: int, n: int) -> int:
+    """Return a non-negative inverse of a to modulo n."""
+    inv = invmod(a, n)
+    return inv if inv >= 0 else inv + n
+
 def hex_to_int(val: str) -> int:
     """Implement the syntax accepted by mbedtls_test_read_mpi().
 
@@ -244,6 +249,8 @@
     #pylint: disable=abstract-method
     """Target for bignum mod_raw test case generation."""
     moduli = MODULI_DEFAULT # type: List[str]
+    mongtomgery_form_a = False
+    disallow_zero_a = False
 
     def __init__(self, val_n: str, val_a: str, val_b: str = "0",
                  bits_in_limb: int = 64) -> None:
@@ -264,6 +271,14 @@
         return self.int_n
 
     @property
+    def arg_a(self) -> str:
+        if self.mongtomgery_form_a:
+            value_a = self.to_montgomery(self.int_a)
+        else:
+            value_a = self.int_a
+        return self.format_arg('{:x}'.format(value_a))
+
+    @property
     def arg_n(self) -> str:
         return self.format_arg(self.val_n)
 
@@ -287,6 +302,8 @@
     def is_valid(self) -> bool:
         if self.int_a >= self.int_n:
             return False
+        if self.disallow_zero_a and self.int_a == 0:
+            return False
         if self.arity == 2 and self.int_b >= self.int_n:
             return False
         return True
diff --git a/scripts/mbedtls_dev/bignum_core.py b/scripts/mbedtls_dev/bignum_core.py
index 1a8c22b..3bd7f11 100644
--- a/scripts/mbedtls_dev/bignum_core.py
+++ b/scripts/mbedtls_dev/bignum_core.py
@@ -757,15 +757,7 @@
     test_function = "mpi_core_exp_mod"
     test_name = "Core modular exponentiation (Mongtomery form only)"
     input_style = "fixed"
-
-    def arguments(self) -> List[str]:
-        # Input 'a' has to be given in Montgomery form
-        mont_a = self.to_montgomery(self.int_a)
-        arg_mont_a = self.format_arg('{:x}'.format(mont_a))
-        return [bignum_common.quote_str(n) for n in [self.arg_n,
-                                                     arg_mont_a,
-                                                     self.arg_b]
-               ] + self.result()
+    mongtomgery_form_a = True
 
     def result(self) -> List[str]:
         # Result has to be given in Montgomery form too
diff --git a/scripts/mbedtls_dev/bignum_mod.py b/scripts/mbedtls_dev/bignum_mod.py
index 9f98131..6428873 100644
--- a/scripts/mbedtls_dev/bignum_mod.py
+++ b/scripts/mbedtls_dev/bignum_mod.py
@@ -58,15 +58,10 @@
     input_style = "fixed"
     arity = 1
     suffix = True
-
-    @property
-    def is_valid(self) -> bool:
-        return self.int_a > 0 and self.int_a < self.int_n
+    disallow_zero_a = True
 
     def result(self) -> List[str]:
-        result = bignum_common.invmod(self.int_a, self.int_n)
-        if result < 0:
-            result += self.int_n
+        result = bignum_common.invmod_positive(self.int_a, self.int_n)
         # To make negative tests easier, append 0 for success to the
         # generated cases
         return [self.format_result(result), "0"]
@@ -80,20 +75,11 @@
     input_style = "arch_split"  # Mont. form requires arch_split
     arity = 1
     suffix = True
-
-    @property
-    def is_valid(self) -> bool:
-        return self.int_a > 0 and self.int_a < self.int_n
-
-    @property
-    def arg_a(self) -> str:
-        mont_a = self.to_montgomery(self.int_a)
-        return self.format_arg('{:x}'.format(mont_a))
+    disallow_zero_a = True
+    mongtomgery_form_a = True
 
     def result(self) -> List[str]:
-        result = bignum_common.invmod(self.int_a, self.int_n)
-        if result < 0:
-            result += self.int_n
+        result = bignum_common.invmod_positive(self.int_a, self.int_n)
         mont_result = self.to_montgomery(result)
         # To make negative tests easier, append 0 for success to the
         # generated cases
diff --git a/scripts/mbedtls_dev/bignum_mod_raw.py b/scripts/mbedtls_dev/bignum_mod_raw.py
index 0084898..461b1f2 100644
--- a/scripts/mbedtls_dev/bignum_mod_raw.py
+++ b/scripts/mbedtls_dev/bignum_mod_raw.py
@@ -64,21 +64,11 @@
     input_style = "arch_split"
     arity = 1
     suffix = True
-
-    @property
-    def is_valid(self) -> bool:
-        return self.int_a > 0 and self.int_a < self.int_n
-
-    @property
-    def arg_a(self) -> str:
-        # Input has to be given in Montgomery form
-        mont_a = self.to_montgomery(self.int_a)
-        return self.format_arg('{:x}'.format(mont_a))
+    mongtomgery_form_a = True
+    disallow_zero_a = True
 
     def result(self) -> List[str]:
-        result = bignum_common.invmod(self.int_a, self.int_n)
-        if result < 0:
-            result += self.int_n
+        result = bignum_common.invmod_positive(self.int_a, self.int_n)
         mont_result = self.to_montgomery(result)
         return [self.format_result(mont_result)]